Thriving on Arsenic

EXTREMOPHILES

With its salt-encrusted shores, alkaline pH, and high levels of arsenic, Searles Lake, in the Mojave Desert, isn't exactly the place for a refreshing summer dip. Although Searles' waters are toxic to people, researchers have discovered a new species of microbe that thrives on the deadly broth, thanks to its ability to generate energy from arsenic (Science 2005, 308, 1305).

The U.S. Geological Survey's Ronald S. Oremland and colleagues at USGS and Pittsburgh's Duquesne University discovered the new bacterium, named SLAS-1. The researchers have observed arsenic's full redox cycle in Searles Lake. In the absence of oxygen, SLAS-1 uses As(V) as an electron acceptor in its respiration process, oxidizing organic matter to CO₂ and reducing the arsenate to As(III). In areas of the lake where there's dissolved oxygen, another organism fixes CO₂ into organic matter, employing As(III) as a source of electrons.

Scientists previously reported other bacteria that use arsenic in a similar manner, but Oremland says he was surprised to find that anything could live in Searles Lake. The water has a pH around 9.8, is saturated with salt, and has a concentration of arsenic about 29,000 times what's acceptable in drinking water. A chemical company extracts borax from the lake's shores.

Despite the harsh conditions, SLAS-1 has adapted well to its environment, Oremland's team found. "It's an extremely interesting organism," he says. In lab experiments, SLAS-1 grows sluggishly if the pH of its surroundings falls below 9.1, and it doesn't grow at all at salinities less than 200 g per L.

According to Oremland, extremophiles like SLAS-1 are of great interest to NASA because they're likely to be similar to microbes that might live in other extreme environments--perhaps on Mars or on Europa, one of Jupiter's moons.